[v2,2/2] ring: move the atomic load of head above the loop

  In __rte_ring_move_prod_head, move the __atomic_load_n up and out of
the do {} while loop as upon failure the old_head will be updated,
another load is costly and not necessary.

This helps a little on the latency,about 1~5%.

 Test result with the patch(two cores):
 SP/SC bulk enq/dequeue (size: 8): 5.64
 MP/MC bulk enq/dequeue (size: 8): 9.58
 SP/SC bulk enq/dequeue (size: 32): 1.98
 MP/MC bulk enq/dequeue (size: 32): 2.30

Fixes: 39368ebfc606 ("ring: introduce C11 memory model barrier option")
Cc: stable@dpdk.org

Signed-off-by: Gavin Hu <gavin.hu@arm.com>
Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
Reviewed-by: Jia He <justin.he@arm.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
---
 doc/guides/rel_notes/release_18_11.rst |  7 +++++++
 lib/librte_ring/rte_ring_c11_mem.h     | 10 ++++------
 2 files changed, 11 insertions(+), 6 deletions(-)
  

31/10/2018 04:35, Gavin Hu:
> --- a/doc/guides/rel_notes/release_18_11.rst
> +++ b/doc/guides/rel_notes/release_18_11.rst
> +* **Updated rte ring C11 driver.**
> +
> +  Updated the rte ring C11 driver including the following changes:

It is not a driver, it is the ring library with C11 memory model.
Please reword and move it after memory related notes (at the beginning).
Thanks
  

> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Wednesday, October 31, 2018 5:37 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>
> Cc: dev@dpdk.org; olivier.matz@6wind.com; chaozhu@linux.vnet.ibm.com;
> bruce.richardson@intel.com; konstantin.ananyev@intel.com;
> jerin.jacob@caviumnetworks.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; stable@dpdk.org
> Subject: Re: [PATCH v2 2/2] ring: move the atomic load of head above the
> loop
>
> 31/10/2018 04:35, Gavin Hu:
> > --- a/doc/guides/rel_notes/release_18_11.rst
> > +++ b/doc/guides/rel_notes/release_18_11.rst
> > +* **Updated rte ring C11 driver.**
> > +
> > +  Updated the rte ring C11 driver including the following changes:
>
> It is not a driver, it is the ring library with C11 memory model.
> Please reword and move it after memory related notes (at the beginning).
> Thanks
>

Done, v3 just submitted, thanks!

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